This invention relates to a device for setting an annular collar on a drill bit so that it is at a predetermined distance from the tip of the drill bit.
Numerically controlled drilling machines are becoming an indispensable fixture in many production shops. For example, they are used to produce circuit boards for electronic equipment. The operation of these drilling machines generally requires compressed air. When the drilling machine is in use, a tray of drill bits travels beneath the drill spindle as directed by computer program instructions, until the correct drill bit is directly beneath the collet of the drill spindle. The collet opens, picks up the selected drill bit, and moves the bit over to a work piece for drilling. Alternatively, the spindle could travel over a stationary tray of drill bits, or both the spindle and the tray can move relative to each other.
For circuit board production, generally the work piece, which is a Fiberglas.TM. plaque to be used as the substrate for the board, is placed on a sheet of backing material generally between about 62 mil to about 125 mil in thickness. The backing material in turn lies flat on an aluminum tooling plate, which is suspended on an air table. It is important that the tip of the drill bit drills down to an accurate depth. The drill bit must not pass through the backing layer and damage the tooling plate. Likewise, a drill hole which is too shallow is unacceptable. A plastic collar is generally placed on the shank of the drill bit at a predetermined distance from the tip of the drill bit. The collar butts against the collet to assure an accurate drill depth.
Drill bits for numerically controlled drilling machines can be factory pre-collared. However, pre-collared bits can be expensive. Moreover, for certain applications the collars have to be set at various distances from the tip of the drill bit during an extended run. It is therefore general practice that the drill bits and collars are purchased separately, with the collars being set on the drill bit by production shop personnel.
Manually operated "Ring Setters" for collaring and decollaring drill bits are popular with most production shops. These devices have a cylindrically shaped barrel. The annular collar, which has an outer diameter larger than the inner diameter of the barrel, is held against one end of the barrel. A drill bit is pushed through the collar and partly into the barrel, where the tip of the drill bit pushes against the contact tip of a mechanical dial indicator. The dial indicator indicates when the collar is set at the correct distance from the tip of the drill bit.
An acceptable accuracy for setting the collar is .+-.5 mils, which the ring setter described above is capable of achieving. However, the existing ring setter suffers from a very serious shortcoming. The end of the drill bit is pushed against the contact tip of the dial indicator. This can chip or otherwise damage the tip of the drill bit, especially with smaller diameter drill bits. The chipping problem is especially serious for drill bits with hardened flutes, e.g., those with flutes formed of a carbide material. Carbide is hard, but it chips easily. It has been estimated that the damage rate can be as high as 20% for carbide drill bits smaller than 1/8 inch in diameter. Replacing or resharpening these damaged drill bits can be expensive and time consuming.
Therefore there is a need for a device for setting a collar on a small diameter drill bit without damaging the drill bit.